1. Field of the Invention
This invention relates to fuel burning. In one of its aspects, the invention relates to a burner which can be fired with a normally liquid fuel, such as fuel oil, or a normally gaseous fuel.
2. State of the Prior Art
In the U.S. pat. Nos. to Gentry 3,706,445 and Phillips 3,670,668, there is disclosed and claimed an incinerator for oxidizing combustible fumes in a mixture of such fumes and air wherein the mixture is heat exchanged with the surface of the combustion chamber to preheat the mixture, the pressure of the mixture is increased in a fan and the mixture is thereafter passed through the combustion chamber wherein the temperature thereof is further raised to an oxidation temperature for the fumes. A flame grid, for example, of the nature described in U.S. Pat. No. to Davies 3,524,632, can be used to raise the temperature of the mixture to the oxidation temperature of the fumes. The flame grid disclosed by Davies is in the nature of a plate having openings for passage of gases therethrough and providing a plurality of protected areas to support combustion thereon. Gaseous fuel is supplied to the protected areas and combustion is initiated thereon so that a flame curtain extends substantially across the plate. The gases passing through the flame curtain come into substantial contact with the fumes to uniformly heat the gases and to provide intimate mixing between the fumes and the combustion zone. The grid system thus maximizes the oxidation of the fumes. The gaseous fuel supplied to the grid is advantageous to the fume incinerator because it can be supplied in gaseous form without the need for addition of external secondary air required for combustion of the fuel. The secondary air is obtained from the air in the mixture of the fumes and air.
The absence of supplying the external secondary air is an important economic consideration in the operation of an incinerator of this type. Use of external or outside fresh air as secondary air for combustion means that additional BTU's must be used to raise the temperature of the fume and air mixture to the incineration temperature which may be 1200.degree.-1400.degree. F. The air added to the system must also be raised to these temperatures by the burning. Thus, considerable savings are afforded by virtue of the fact that no additional air is required to be added to burn the fuel.
Recently, supply of natural gas has been short, particularly during periods of peak use. New applications for use of natural gas for industrial sources have been denied or delayed because of the tight supply. In addition, industry use of natural gas during peak periods has been limited because of typical bulk-rate contract provisions. It is therefore necessary that other sources of energy be available in incinerators of this type.
Liquid fuel, such as industrial heating oil, is suitable for use in industrial incineration. However, such a fuel cannot be readily substituted for natural gas in natural gas burners, such as disclosed in the Davies patent. Liquid fuels, such as oils, require special systems for supplying the fuel to a combustion chamber. Normally, the fuel is required to be atomized and mixed with sufficient secondary air for burning. However, use of conventional oil burners with additional secondary air would add considerable cost to the incineration process because of the heating expense for the external air used in burning. Further, the liquid fuel, in atomized form or otherwise, cannot be simply forced through the gas burner because the flow parameters are different from the oil and gas systems.
Normally, a mixture of fumes and air cannot be used in heated form as the secondary air supplied to a conventional oil burner. The fumes, as for example from a drying oven, contain resins and similar materials which would be likely to deposit on the rotor blades of blowers and on ports of the burners at low temperatures, resulting in malfunction of the burner. Further, the oil burners are normally operated at a low temperature and the heated air would not work in such conventional burners. Special burners would have to be designed for use in these types of incinerator systems.
Hardison et al., U.S. Pat. No. 3,484,189, disclose an apparatus for mixing fuel with a fume and air mixture without the addition of secondary air. Hardison et al. introduce fuel into the central portion of a frusto conical mixture having a plurality of openings through which air passes for mixing with the fuel. The fuel used in the system apparently is a gaseous fuel and combustion is initiated as the fuel is introduced into the frusto conical mixture. In this system, only a portion of the contaminated air mixes with the fuel and the remainder of the gases bypass the combustion zone. The possibility exists that some of the fumes may not be heated sufficiently for oxidation of the fumes. As stated above, oxidation of the fumes is more certain when the fumes are passed through a flame front or flame curtain which heats the fumes to an oxidation temperature.